As illustrated in U.S. Pat. No. 5,052,462, a canvas tensioning picture frame is illustrated in which a frusto-conical screw is passed through two disks, with the screw being rotated to force the abutting frame members apart, thereby to provide tensioning. While this two disk structure works relatively well to provide a stable structure for the use of canvas tensioning, it nonetheless requires two disks carried in respective slots in the abutting frame members to provide for the relative stability of the frame.
It was thought that the two disks were necessary to provide lateral and longitudinal stability when the abutting frame members were moved apart to tension the canvas. The lateral and longitudinal stability is necessary to prevent wrinkling of the canvas or warping of the frame members with respect to each other. Moreover, as described in this patent, the first of the disks through which the screw passes has raised ribs and a square aperture, such that the screw is captured in the disk and is rotated by virtue of rotating the disk. In this design, there is a trade off between the tightness with which the first disk is carried by the abutting frame members and the ability to rotate the screw. The result is that the first disk is made tight within its slot to preserve the dimensional stability of the framing system, which tightness precludes easy turning of the tapered screw.
Thus, for structural stability the first disk must be quite tight within its slot. However, its rotation is difficult and also disturbs the alignment of the abutting frame members. While this canvas stretcher has worked quite satisfactorily in the past, there is an opportunity for significant structural and cost improvements.
By way of general background, it will be appreciated that an artist's canvas is, in general, held stretched tight upon a stretcher bar frame. The requirements of such a frame are that it should be optimally rigid against torsion and twisting, dimensionally stable during temperature or humidity changes, light weight and inexpensively constructed of readily available materials. Typically, and as described in the aforementioned patent, the frame is made of wood with mitered corners which may exhibit duck-tail or other types of interlocking corner joints. Over prolonged time periods deformation of the hand stretched canvas frame is induced by longitudinal and lateral stresses arising from the canvas or from the environment. These stresses can cause deviations from the desired angle between the frame members, normally a right angle and/or a twisting or warping of the frame in its established plane. Such warpage or twisting of the frame typically causes stress to be developed in the canvas, which after prolonged duration, results in wrinkles be generated in the canvas. In an attempt to remedy such wrinkles, it has long been known to manually drive wooden wedges between adjacent ends of the frame members at the mitered corners of the frame, which cause a selective spreading of the frame members to re-stretch the canvas thereon.
Such wooden wedges, however, typically weaken the comer joint integrity and often result in further frame warpage, thereby perpetuating the problem. In addition, the manual driving of the wedges into the comers of the frame often causes inadvertent cutting or tearing of the canvas due to abrasion of the wedge upon rear surface of the canvas. Such inadvertent weakening, of course, results in permanent, non-recoverable damage to the artist's rendition on the canvas. In view of the recognition of this problem, certain framing solutions involving a long term tensioning a canvas stretched upon a frame has been sought. Particularly, certain frames which constantly tension the canvas stretched upon such frames have been developed in the prior art. One such wedge type solution is shown in U.S. Pat. No. 1,128,362; whereas a screw actuated tensioning device is shown in U.S. Pat. No. 2,149,985.
In the past, camming devices have been utilized in between adjacent and abutting frame members such as illustrated in U.S. Pat. No. 3,238,996, 371,933 and 371,934. Of particular interest is U.S. Pat. No. 4,829,685 in which a camming member, when rotated, bears upon an arcuate hollow portion of an adjacent frame member, with the screw bearing upon an opposite semi-circular cut in the abutting frame member.
It will be appreciated that in all of the above noted background patents, the structural stability of the frame doing tensioning or otherwise is in question. In general, it has been found by the prior art that the structural stability needs to be increased such as through the utilization of pegs, elbows, screws, or complicated mechanical stability producing devices. Thus, for instance, in the aforementioned Pat. No. 5,052,462, a double disk structural rigidity reinforcing structure was deemed necessary.
By way of further background, one prior art frame of this type is the "GOLDLINE" constant tension, stretched-canvas frame available from H. W. Peel & Co., Ltd., Norwester House, Fairway Estate, Fairway Drive, Greenford, Middlesex, UB6 8PW, United Kingdom. This frame uses wood peripheral frame members which are mitered to approximately 45 degrees at their comers. An elongate metal spring clip is positioned between adjacent frame members at the junction of their mitered comers. This spring clip tends to force the adjacent frame members apart from each other. The frame members are not directly or rigidly affixed to each other. Rather, in order to hold each pair of adjacent frame members together in their desired spatial relationship defining a frame, a plastic comer piece is used at each comer joint. The plastic comer piece slidably engage adjacent wooden frame members and lie over the top of the centrally positioned spring clip. The plastic comer pieces attempt to hold the wooden frame members, which are being forcibly spread apart by the spring force generated in the intervening spring clip, in positional alignment even though the frame members are not in direct contact with one another. In maintaining this positional alignment, the plastic comer pieces attempt to suppress the torsional, longitudinal and vertical movements which might otherwise result in a distortion of the frame. The plastic comer pieces further attempt to prevent twisting of adjacent frame members relative to one another, and to thereby prevent any resultant distortion of the plane of the canvas established by such frame. As will be discussed after further consideration of the construction of this prior art constantly tensioning, stretched-canvas frame, these attempts are not completely successful.
In order to assemble the "GOLDLINE" constant tension, stretched-canvas frame, the canvas is stretched over, and tacked to, the sides of the peripheral frame members while the frame members are held in alignment by the plastic comer pieces. The metal spring clips are then positioned between the mitered comers of adjacent frame embers, thereby spreading the frame members apart and tensioning the canvas. In order that even further additional tensioning forces may be applied, it is also known in this prior art frame to position a central cross members consisting of two perpendicular arms within the rear interior region of the frame. Each arm of the cross member spans between an opposed pair of frame members.
Each of four ends of the two cross member arms do not directly abut the frame member adjacent its end, but rather contact the frame member through additional intervening spring clips. Since the wooden pieces of the peripheral frame members and of the cross members are not directly connected, plastic guide pieces are used to maintain required alignments. The additional cross member, plastic comer pieces and the plastic guide pieces undesirably add considerable weight and cost to the frame.
This particular "GOLDLINE" prior art stretcher frame is utilized to continuously present tensioning forces to the canvas which is stretched taut upon it. However, in order to obtain this tensioning, and the required "full-floating" relationship between all frame members, this prior art tensioning frame incurs a great penalty in the establishment and maintenance of a precision alignment between and among the frame members. The plastic comer pieces must slidably engage the fame members over a large surface area, on the order of several square inches, in order to obtain an adequate grasp on such frame members for the purpose of establishing and maintain their relative alignment. However, both the frame pieces and the plastic comer pieces are poorly adaptable to precision construction. Furthermore, the dimensions of the wooden frame members vary with temperature, humidity, and age relative to the retaining plastic comer pieces.
These prior art comer plastic pieces, which slidably engage the frame members in order to guide them into alignment, have therefore proven to be generally inadequate for this task. Particularly, the "GOLDLINE" prior art stretcher frame does not exhibit an alignment between frame members which is as equivalently rigid, precise or permanent to the alignment routinely attained by prior art fixed, non-tensioning frames with rigid comer joints.